The present invention relates, in general, to the field of video display generators. More particularly, the present invention relates to a system and method for altering an aspect, for example color shading, of one of a plurality of coincident visual objects on a video display screen.
Generally, in a sophisticated microprocessor (MPU)-based video display, numerous background or "fixed" objects may be displayed. Such fixed objects are especially useful in video games and other graphics applications. Fixed objects are generally derived from a display system "screen memory" which generally comprises a block of DRAM, the full contents of which would describe the "visible screen".
In such video display systems, a user can display objects by individual "pels", or picture elements, in a "bit-plane mode", or fixed objects in a "list mode". The screen memory in the bit plane mode may be arranged in scan lines. within each scan line, the color of the first pel is followed by the color of the second, etc. The fixed objects in a list mode are defined in "image tables" which contain their pel-by-pel descriptions. A list mode screen memory is arranged as a "display list" of pointers to entries in the image tables, which include "attributes" of the images. These attributes allow each individual occurrence of an image to be altered, by for example, underlining or flashing.
"True objects" are video display generator hardware-intensive objects which are designed to move around on the video display screen and to run into, or collide with, other true or fixed objects. Such true objects comprise a small video pattern which is stored in a particular location in the memory of the video display generator. These true objects can be positioned on the screen independently of where they are stored in memory merely by defining their vertical and horizontal position. By use of true objects, a video display screen may be easily animated and the object stored in the particular location in memory may be made to move across the screen merely by changing its start address.
In such video display systems, it would therefore be useful to display an interaction between true objects and fixed objects. For example, when a true object overlaps a fixed object, the color of the highest priority object can be modified such that it appears that the true object has entered the shadow of the fixed object, becomes illuminated by a light source comprising a fixed object, etc. The use of software to effectuate such a display alteration would require extensive MPU utilization, virtually tying up the processing unit. It would therefore be highly desirable to implement such an object aspect altering system by means of readily integratable hardware which would relieve the system MPU from such "collision" or coincidence computations.